[WIP] Add dimension scales/coordinates

[rly]

Add dimension scales/coordinates to support direct association of datasets with dimensions of other datasets within a group. See https://github.com/NeurodataWithoutBorders/pynwb/issues/626

Very much a work in progress. I am also taking the opportunity to comment and refactor src/hdmf/build/map.py. ObjectMapper is now in its own 1000-line file. Unfortunately, the refactoring obscures the changes I made in the ObjectMapper.